1. Technical Field
The present invention relates to a radar apparatus for transmitting and receiving radar waves to thereby detect an object reflecting the radar waves.
2. Related Art
A known vehicle-mounted radar apparatus transmits radar waves, such as laser waves or millimeter waves, with a predetermined scan angle every predetermined time interval and receives the radar waves reflected from an object around the vehicle mounting the radar apparatus thereon (hereinafter referred to as the own vehicle), thereby detecting the object around the own vehicle.
Such a vehicle-mounted radar apparatus is used in an automatic cruise control (ACC) system or the like adapted to detect a vehicle traveling ahead of the own vehicle in the same lane as the own vehicle (referred to as a preceding vehicle), and control a vehicle speed of the own vehicle so that a distance between the own vehicle and the preceding vehicle is maintained or kept constant.
To prevent a vehicle traveling in a lane other than the own lane (the lane in which the own vehicle is traveling) from being detected incorrectly as a preceding vehicle during cornering of the own vehicle, the ACC system mounted on the own vehicle, as disclosed in Japanese Patent Application Laid-Open Publication No. H08-27909, calculates a probability that a vehicle ahead of the own vehicle is traveling in the own lane on the basis of a curve radius of the own lane and a position of the vehicle traveling ahead of the own vehicle relative to the own vehicle, and on the basis of the calculated probability, determines whether or not the vehicle ahead of the own vehicle is a preceding vehicle traveling in the own lane. In the ACC system, when it is determined that the vehicle ahead of the own vehicle is a preceding vehicle, a distance between the preceding vehicle and the own vehicle is controlled to be maintained or kept constant.
The preceding vehicle may include not only reflection points on the rear portion of the preceding vehicle, but also reflection points in the interior of the preceding vehicle, as shown in FIG. 6A (where, for simplicity, only one reflection point on the rear portion is depicted and each of the other reflection points is in the interior or on a side portion of the preceding vehicle). The term “reflection point” refers to a point or spot from which radar waves are reflected. Mostly, a position of the preceding vehicle relative to the own vehicle is determined by using the reflection point(s) on the rear portion of the preceding vehicle.
As shown in FIG. 6B, however, a lateral position (a position along a width of the own vehicle) of the reflection point on the rear portion of the preceding vehicle may be off-centered even when the preceding vehicle is traveling straight. This is because, since the preceding vehicle is unable to travel completely straight, an aspect angle between the preceding vehicle and the own vehicle varies with time, or pitching of the own vehicle causes a back-and-forth rocking motion, the position of the reflection point on the rear portion of the preceding vehicle fluctuates due to the nature of millimeter waves. In addition, during cornering of the preceding vehicle as shown in FIG. 6C, the lateral position of the reflection point on the rear portion of the preceding vehicle may be displaced in a cornering direction from the lateral position before the cornering. That is, the lateral position of the reflection point on the rear portion of the preceding vehicle may be displaced to the left for a left-hand curve or to the right for a right-hand curve. This is because, when the preceding vehicle is traveling straight, radar waves reflected from the center or its vicinity of the rear portion of the preceding vehicle will be detected by the own vehicle, and when the preceding vehicle is cornering to the left/right, radar waves reflected from a left/right edge of the rear portion of the preceding vehicle will be detected by the own vehicle. As such, whether during traveling straight of the preceding vehicle or during cornering of the preceding vehicle, the detected position of the preceding vehicle may thus vary with time. Accordingly, the preceding vehicle traveling in the same lane as the own vehicle may be detected incorrectly as a vehicle traveling in a different lane.
Particularly, when the preceding vehicle is a large vehicle, such as a truck or the like, a vehicle width is greater than that of a mid-size car. Accordingly, the large vehicle has a distance between the center of the rear portion and the left/right edge of the rear portion greater than that of the mid-size car. This may lead to a greater reduction in accuracy of detecting the position of the preceding vehicle.
In consideration of the foregoing, it would therefore be desirable to have a vehicle-mounted radar apparatus capable of preventing reduction in accuracy of detecting a position of a preceding vehicle due to the preceding vehicle being a large vehicle.